This changeset fixes a common problem for busy networking kthreads.
These threads, e.g. NAPI threads, typically will do:

* polling a batch of packets
* if there are more work, call cond_resched to allow scheduling
* continue to poll more packets when rx queue is not empty

We observed this being a problem in production, since it can block RCU
tasks from making progress under heavy load. Investigation indicates
that just calling cond_resched is insufficient for RCU tasks to reach
quiescent states. This at least affects NAPI threads, napi_busy_loop, and
also cpumap kthread for now.

By reporting RCU QSes in these kthreads periodically before
cond_resched, the blocked RCU waiters can correctly progress. Instead of
just reporting QS for RCU tasks, these code share the same concern as
noted in the commit d28139c4e967 ("rcu: Apply RCU-bh QSes to RCU-sched
and RCU-preempt when safe"). So report a consolidated QS for safety.

It is worth noting that, although this problem is reproducible in
napi_busy_loop, it only shows up when setting the polling interval to as
high as 2ms, which is far larger than recommended 50us-100us in the
documentation. So napi_busy_loop is left untouched.

V2: https://lore.kernel.org/bpf/ZeFPz4D121TgvCje@debian.debian/
V1: https://lore.kernel.org/lkml/Zd4DXTyCf17lcTfq@debian.debian/#t

changes since v2:
 * created a helper in rcu header to abstract the behavior
 * fixed cpumap kthread in addition

changes since v1:
 * disable preemption first as Paul McKenney suggested

Yan Zhai (3):
  rcu: add a helper to report consolidated flavor QS
  net: report RCU QS on threaded NAPI repolling
  bpf: report RCU QS in cpumap kthread

 include/linux/rcupdate.h | 23 +++++++++++++++++++++++
 kernel/bpf/cpumap.c      |  2 ++
 net/core/dev.c           |  3 +++
 3 files changed, 28 insertions(+)

-- 
2.30.2

Yan Zhai <yan@cloudflare.com> writes:

> This changeset fixes a common problem for busy networking kthreads.
> These threads, e.g. NAPI threads, typically will do:
>
> * polling a batch of packets
> * if there are more work, call cond_resched to allow scheduling
> * continue to poll more packets when rx queue is not empty
>
> We observed this being a problem in production, since it can block RCU
> tasks from making progress under heavy load. Investigation indicates
> that just calling cond_resched is insufficient for RCU tasks to reach
> quiescent states. This at least affects NAPI threads, napi_busy_loop, and
> also cpumap kthread for now.
>
> By reporting RCU QSes in these kthreads periodically before
> cond_resched, the blocked RCU waiters can correctly progress. Instead of
> just reporting QS for RCU tasks, these code share the same concern as
> noted in the commit d28139c4e967 ("rcu: Apply RCU-bh QSes to RCU-sched
> and RCU-preempt when safe"). So report a consolidated QS for safety.
>
> It is worth noting that, although this problem is reproducible in
> napi_busy_loop, it only shows up when setting the polling interval to as
> high as 2ms, which is far larger than recommended 50us-100us in the
> documentation. So napi_busy_loop is left untouched.
>
> V2: https://lore.kernel.org/bpf/ZeFPz4D121TgvCje@debian.debian/
> V1: https://lore.kernel.org/lkml/Zd4DXTyCf17lcTfq@debian.debian/#t
>
> changes since v2:
>  * created a helper in rcu header to abstract the behavior
>  * fixed cpumap kthread in addition
>
> changes since v1:
>  * disable preemption first as Paul McKenney suggested
>
> Yan Zhai (3):
>   rcu: add a helper to report consolidated flavor QS
>   net: report RCU QS on threaded NAPI repolling
>   bpf: report RCU QS in cpumap kthread
>
>  include/linux/rcupdate.h | 23 +++++++++++++++++++++++
>  kernel/bpf/cpumap.c      |  2 ++
>  net/core/dev.c           |  3 +++
>  3 files changed, 28 insertions(+)

For the series:

Reviewed-by: Toke Høiland-Jørgensen <toke@redhat.com>

On 13/03/2024 17.25, Yan Zhai wrote:
> This changeset fixes a common problem for busy networking kthreads.
> These threads, e.g. NAPI threads, typically will do:
> 
> * polling a batch of packets
> * if there are more work, call cond_resched to allow scheduling
> * continue to poll more packets when rx queue is not empty
> 
> We observed this being a problem in production, since it can block RCU
> tasks from making progress under heavy load. Investigation indicates
> that just calling cond_resched is insufficient for RCU tasks to reach
> quiescent states. This at least affects NAPI threads, napi_busy_loop, and
> also cpumap kthread for now.
> 
> By reporting RCU QSes in these kthreads periodically before
> cond_resched, the blocked RCU waiters can correctly progress. Instead of
> just reporting QS for RCU tasks, these code share the same concern as
> noted in the commit d28139c4e967 ("rcu: Apply RCU-bh QSes to RCU-sched
> and RCU-preempt when safe"). So report a consolidated QS for safety.
> 
> It is worth noting that, although this problem is reproducible in
> napi_busy_loop, it only shows up when setting the polling interval to as
> high as 2ms, which is far larger than recommended 50us-100us in the
> documentation. So napi_busy_loop is left untouched.
> 
> V2: https://lore.kernel.org/bpf/ZeFPz4D121TgvCje@debian.debian/
> V1: https://lore.kernel.org/lkml/Zd4DXTyCf17lcTfq@debian.debian/#t
> 
> changes since v2:
>   * created a helper in rcu header to abstract the behavior
>   * fixed cpumap kthread in addition
> 
> changes since v1:
>   * disable preemption first as Paul McKenney suggested
> 
> Yan Zhai (3):
>    rcu: add a helper to report consolidated flavor QS
>    net: report RCU QS on threaded NAPI repolling
>    bpf: report RCU QS in cpumap kthread
> 
>   include/linux/rcupdate.h | 23 +++++++++++++++++++++++
>   kernel/bpf/cpumap.c      |  2 ++
>   net/core/dev.c           |  3 +++
>   3 files changed, 28 insertions(+)

Acked-by: Jesper Dangaard Brouer <hawk@kernel.org>